More of a note-to-self than anything, but potentially useful if you use Mendeley:
Mendeley searches were becoming quite slow for me, even with a database of just over a thousand items. It would take more than 30 seconds for a simple query.
I fixed this problem by having Mendeley rebuild its database. That database can be found by opening up the software, pressing Ctrl+Shift+D to open the debug console, then clicking the Open Data Directory button. I opened the folder "www.mendeley.com" and the folder inside it named with my email address. That's where the sqlite database resides. Close Mendeley, remove the database files (mine had ballooned to more than a gigabyte - not sure if that's normal for this software) and start up Mendeley again.
June 20, 2019 edit: This is also a good solution if Mendeley crashes upon attempting to sync. Apparently its usual response to database corruption of any kind is to crash.